1. Field of the Invention
The present invention relates to infrared thermometers, more specifically to a thermometer which uses a shutter or chopper for a thermal reference.
2. Description of the Prior Art
A wide choice of sensors are available for measurement of the temperature of an object by infrared radiation. The range of choice includes but is not limited to thermopiles, pyroelectrics, bolometers, and active infrared sensors.
An infrared sensor generates a signal which is representative of two temperatures: its own surface temperature and that of the object target. A relationship between those temperatures and a sensor's reaction is governed by Stefan-Boltzmann law: EQU V=k.epsilon..sub.b .epsilon..sub.s (T.sub.b.sup.4 -T.sub.s.sup.4)(1)
where V is the sensor's output signal, T.sub.s its surface temperature .epsilon..sub.b and .epsilon..sub.s are emissivities of the target and sensor respectively, k is a constant, and T.sub.b is the temperature of the object.
It is seen from the above equation that to calculate T.sub.b, one must first determine two values: V and T.sub.s. This is not a simple task because an infrared sensor with a good speed response is generally fabricated in the form of a thin flake or membrane, of which the surface temperature is difficult to measure and changes upon exposure to a target. Inaccurate determination of a sensor's surface temperature results in error in a non-contact temperature sensor. Several methods for reducing the error are known to the art.
One way to minimize the error is to first measure thermal radiation from a known temperature source and then from the object.
The sensor is alternatively exposed to the calibrated temperature and then to the unknown temperature. This may be accomplished by use of a chopper as described in U.S. Pat. No. 4,907,895 issued to Everest or by use of a shutter as in U.S. Pat. No. 4,797,840 issued to Fraden.
U.S. Pat. No. 4,634,294 issued to Christol et al. teaches use of a folded mirror, while U.S. Pat. No. 4,005,605 issued to Michael describes a moving focusing mirror to aim the infrared sensor alternatively toward the reference target and toward the object target.
The temperature of the mechanical chopper or shutter is determined by reference to a separate temperature sensor which measures ambient temperature.
Given the value of the ambient temperature, the temperature of the object target may be calculated from the equation: EQU V=.epsilon..sub.b .epsilon..sub.s (T.sub.b.sup.4 -T.sub.a.sup.4)(2)
where T.sub.a is the ambient temperature. It is seen now that T.sub.s is not a part of the calculation. This implies that T.sub.s can be an unimportant factor in accurate measurement of temperature. This possibility can be closely approached if a shutter, chopper or mirror alternates infrared flux at the infrared sensor surface relatively quickly to prevent drift in the infrared sensor's surface temperature.
Adjusting equation 2 for calculating the temperature of an object target, we obtain equation 3, where n is a constant that is determined during the calibrating process by measuring the temperature of a calibrated heat source of a known temperature: ##EQU1##
Another way to reduce the error associated with un-known T.sub.s is described in U.S. Pat. No. 4,854,730, issued to Fraden where the sensor's surface temperature is actively controlled to stay constant at a predetermined level. The method is carried out without moving components, but requires that the selected T.sub.s be generally higher than the highest expected temperature of the object target. This can be a limiting factor in some applications.
U.S. Pat. Nos. 4,790,324 and 4,602,642 issued to O'Hara et al. teach another method of solving the problem in which no mechanical chopper or shutter is required during measurement of the object. A separate calibrated reference target, however, is employed. The temperature of that calibrated target is controlled and accurately measured.
Before measurement of the object temperature, the infrared sensor probe of the thermometer is manually positioned to view the calibrated target to detect its infrared flux and calibrate the infrared sensor. To further improve accuracy, the infrared sensor body is heated to bring its temperature closer to that of the reference target. This reduces sensor errors, such as drift, non-linearity, noise, etc.
Disadvantages of this method include complex design, high power consumption and need of the manual calibration before each temperature measurement.
A pyroelectric infrared thermometer which is described by U.S. Pat. No. 4,797,840 issued to Fraden includes a shutter for causing a transient response in the infrared sensor. The pyroelectric type sensor is an a.c. device which responds to change in temperature, rather than to its temperature state. One example is an ultra thin foil of pyroelectric material such as polyvinylidene fluoride (PVDF). If electrically polarized, such a film exhibits a pyroelectric effect in that it is able to generate an electrical charge in response to a change in its temperature produced by the receipt of infrared radiation.
In Fraden '840, the shutter in the pyroelectric thermometer serves two functions. One is to control infrared radiation flow to the infrared sensor to generate a transient response. The other is to serve as a thermal reference source at ambient temperature that provides intimate thermal coupling with the sensor within a period that the sensor is not exposed to the object. The sensor is exposed to the surface of the blade prior to measurement of the object, and ambient temperature is measured by a second sensor to obtain the reference temperature for use in calculations.
One major disadvantage of prior art moving-part thermometers is a relatively broad range of a shutter or chopper temperatures resulting from their following ambient conditions. In order to overcome this deficiency, it is necessary to use more costly infrared sensors of higher stability and low noise, and electronic processing circuits of broad dynamic range. The instrument therefore becomes more complex in design and expensive to manufacture.